1. Field of the Invention
The present invention relates generally to magnetoresistive elements and magnetic heads for reproducing information in a magnetic storage unit, magnetic storage units, and magnetic memory units, and more particularly to a magnetoresistive element having a CPP (Current-Perpendicular-to-Plane) structure where sense current is caused to flow in the stacking direction of the stacked films forming the magnetoresistive element.
2. Description of the Related Art
In recent years, in the magnetic head of a magnetic storage unit, a magnetoresistive element has been employed as a reproduction element for reproducing information recorded in a magnetic recording medium. The magnetoresistive element reproduces information recorded in the magnetic recording medium using the magnetoresistance effect converting a change in the direction of a signal magnetic field leaking out from the magnetic recording medium into a change in electric resistance.
With the increase in the recording density of the magnetic storage unit, magnetoresistive elements with a spin-valve film have become mainstream. The spin-valve film includes a fixed magnetization layer in which magnetization is fixed in a predetermined direction, a non-magnetic layer, and a free magnetization layer in which the direction of magnetization changes in accordance with the direction and the strength of a leakage magnetic field from a magnetic recording medium. The electric resistance of the spin-valve film changes in accordance with the angle formed by the magnetization of the fixed magnetization layer and the magnetization of the free magnetization layer. The magnetoresistive element reproduces bits recorded in the magnetic recording medium by detecting a change in the electric resistance as a change in voltage by causing a sense current of a fixed value to flow through the spin-valve film.
Conventionally, a CIP (Current-In-Plane) structure where sense current is caused to flow in the in-plane direction of a spin-valve film has been employed for the magnetoresistive element. In order to further increase the recording density of the magnetic recording unit, it is necessary in the magnetic recording medium to increase its track recording density and track density. On the other hand, in the magnetoresistive element, it is necessary to reduce both element width corresponding to the track width of the magnetic recording medium and element height (the depth of the element), that is, the element cross section. In this case, in the CIP structure, the current density of sense current increases so that the degradation of performance may be caused because of the migration of a material forming the spin-valve film due to overheating.
Therefore, a CPP structure where sense current is caused to flow in the stacking direction of the spin-valve film, that is, the direction in which the fixed magnetization layer, the non-magnetic layer, and the free magnetization layer are stacked, has been proposed and studied eagerly as a next-generation reproduction element. The CPP-type spin-valve film, which has the merit that narrowing element width hardly changes output, is suitable for increasing recording density.
The output of the CPP-type spin-valve film is determined by a change in magnetoresistance at the time of applying an external magnetic field to the spin-valve film by a magnetic field sweep from one direction to the opposite direction. This change in magnetoresistance is the magnetoresistance change of the unit area of the film surface perpendicular to the direction of the sense current. The magnetoresistance change of the unit area is the product of the magnetoresistance change of the spin-valve film and the film surface of the spin-valve film. In order to increase the magnetoresistance change of the unit area, a material whose product of a spin-dependent bulk scattering coefficient and specific resistance is large should be used for the free magnetization layer and the fixed magnetization layer. The spin-dependent bulk scattering coefficient indicates the degree to which conduction electrons scatter inside the free magnetization layer or the fixed magnetization layer depending on the spin orientations of the conduction electrons. The greater the spin-dependent bulk scattering coefficient, the greater the magnetoresistance change.
For example, Japanese Laid-Open Patent Application No. 2003-218428 (hereinafter, JP2003-218428) proposes a magnetoresistive element using, as a material of a large spin-dependent bulk scattering coefficient, a soft magnetic alloy of a Heusler alloy composition for the free magnetization layer.
However, in the case of using Co2MnAl of a Heusler alloy composition for the free magnetization layer as in JP2003-218428 described above, its high coercive force slows the response of the magnetization of the free magnetization layer to the signal magnetic field from a magnetic recording medium. That is, the sensitivity of the magnetoresistive element is reduced. There is a general tendency for the signal magnetic field strength from a magnetic recording medium to decrease as the recording density increases. Accordingly, if the coercive force of the free magnetization layer is high, the electric resistance due to the magnetoresistance effect may not be saturated. This reduces a substantial change in magnetoresistance, thus decreasing the output of the magnetoresistive element. Further, if the coercive force is too high, the magnetization of the free magnetization layer hardly rotates because of the signal magnetic field, so that little output may be obtained.